In 1967 Daniel Schwartz and Joseph Lellouch1 argued that there are 2 kinds of randomised controlled trials (RCTs) embodying radically different attitudes to evaluation of treatment. They named these trials “pragmatic” and “explanatory” and stated that these 2 attitudes require different approaches to the design of an RCT. The pragmatic attitude seeks to directly inform real-world decisions among alternative treatments, and Schwartz and Lellouch show that this purpose is satisfied in trials that test feasible interventions on typical patients in common settings, with usual care as the comparator, to widen real-world applicability. The explanatory attitude, in contrast, is directed to understanding a biological process by testing the hypothesis that the specified biological response is explained by exposure to a particular treatment. Tight restrictions on eligible participants, intense and closely monitored treatment, inactive control interventions (such as placebo), and an idealised healthcare setting maximise the comparison between intervention and control groups and increase the ability to test this kind of hypothesis.

What attitude to RCT design is most useful for patients and clinicians? Clearly, the trial has to ask an important question that is relevant to some aspect of the care clinicians provide to their patients. The clinicians and patients in the trial should resemble the clinicians who are reading the trial report and the patients they typically treat. The intervention being evaluated in the trial should be deliverable by the clinician, and the outcome being used to judge whether the intervention is effective has to be something that the clinician and his or her patients recognise as being worth influencing. In short, the trial has to be applicable, or have what is often called external validity.2

Consider the NASCET trial.3 It asked the following question: among patients with symptomatic 70–99% stenosis of a carotid …